Compressible air gap means compensating for thermal expansion of a lighting magnet coil

ABSTRACT

Lengths of sealed plastic tubing are positioned circumferentially of a lifting magnet coil prior to its encapsulation in a magnet case. The tubing provides an air space at the outer periphery of the magnet coil to prevent damage to the magnet by permitting thermal expansion of the coil without substantial force being applied through the encapsulating material.

' United States Patent [1 1 Schurr [451 Oct. 2, 1973 COMPRESSIBLE AIR GAP MEANS COMPENSATING FOR THERMAL EXPANSION OF A LIGHTING MAGNET COIL [75] Inventor: Charles Allan Schurr, Shaker Heights, Ohio [73] Assignee: Square D Company, Park Ridge, 111.

[22] Filed: Feb. 22, 1972 [21] Appl. N0.: 228,077

[52] 0.8. CI 335/292, 335/294, 294/655 [51] int. Cl H0" [58] Field of Search 335/217, 289, 291,

[56] References Cited UNITED STATES PATENTS 3,693,126 9/1972 Rybak 335/291 950,718 3/1910 Eastwood 335/291 FOREIGN PATENTS OR APPLICATIONS 575,472 5/1959 Canada 335/289 Primary Examiner-George Harris Attorney-Harold J. Rathbun et al.

[5 7 1 ABSTRACT Lengths of sealed plastic tubing are positioned circumferentially of a lifting magnet coil prior to its encapsulation in a magnet case. The tubing provides an air space at the outer periphery of the magnet coil to prevent damage to the magnet by permitting thermal expansion of the coil without substantial force being applied through the encapsulating material.

5 Claims, 3 Drawing Figures Patented Oct. 2, 1973 FIG.B

l HHTIHHH I COMPRESSIBLE AIR GAP MEANS COMPENSATING FOR THERMAL EXPANSION OF A LIGHTING MAGNET COIL This invention relates to lifting magnets and, more particularly, to an improved means and method for protecting component parts of a lifting magnet from cracking, buckling or being otherwise damaged due to internal stresses resulting from thermal expansion of the lifting magnet coil.

Lifting magnets are constructed by placing a wound magnet coil of aluminum or copper into a cavity formed in a cast steel magnet case with leads from the magnet coil extending through an opening in the case to accommodate energization of the coil. The case is closed by a bottom plate welded or otherwise secured to the magnet case and the magnet coil is encapsulated by a thermosetting insulating material to improve the insulation and to enhance the heat dissipation qualities of the magnet by filling all voids in the magnet case.

During operation of a lifting magnet, large currents pass through the magnet coil. The extensive heating of the coil resulting from these currents causes significant expansion of the coil. The expansion of the coil is transmitted through the solid layer of encapsulating material 7 to the magnet case and bottom plate. Due to thermal and material differences, the magnet case does not expand as much as the coil so that the encapsulating material is subjected to great compressive forces. Due to the relative incompressibility of the encapsulating material, the magnet coil is held against expansion and subjected thereby to great stresses.

The high magnitude forces may be sufficient to break the weld sealing the bottom of the magnet. Upon subsequent cooling of the magnet, contraction of the magnet coil may draw moist air into the magnet. The moisture could then cause extensive damage to the lifting magnet as is well known to those skilled in the art. Additionally, the forces applied to the coil by the encapsulating material during the'numerous expansions and contractions accompanying alternate heating and cooling of the magnet coil within this enclosed space could cause extensive damage to the coil itself.

This problem has long been recognized. US. Pat. No. 950,718, issued to Arthur C. Eastwood on Mar. 1, 1910, teaches leaving an air space above the encapsulating compound to take up expansion of the compound caused by heating of the magnet coil. However, at that time, magnets were encapsulated with materials, such as asphalt, which softened with heating and would thus flow into the air space provided. Currently used potting compounds, such as epoxy and tung oil, harden by polymerizing and do not soften upon subsequent heating. Therefore, this concept of the Eastwood invention would be inoperative with present day materials.

It is known to introduce small air spaces during encapsulation of electrical equipment by mixing air-filled objects, such as small spheres, into the encapsulating material prior to encapsulation. However, this procedure is expensive, time consuming and difficult and provides no assurance that the air spaces will be distributed homogeneously throughout the encapsulating material. A non-homogeneous distribution could provide excessive air space and result in inadequate heat dissipation at one portion of the magnet coil while leaving no air space for expansion stress relief at another portion of the coil. Obviously, such a distribution could increase magnet damage instead of preventing it. Additionally, the distribution of air gaps throughout the encapsulating material may greatly decrease the thermal conductivity of the encapsulating material so that the temperature of the magnet coil will become higher during energization.

It is an object of this invention to provide an improved means for, and method of, protecting a lifting magnet from damage due to expansion caused by heating of the magnet coil by providing an air space of a controlled size and location within the magnet.

It is a further object of this invention to provide an improved means for protecting lifting magnets from damage due to expansion caused by heating of the magnet coil without significant reduction of the thermal conductivity of the encapsulating material.

It is a still further object of the present invention to provide an improved means for protecting lifting magnets from damage due to expansion caused by heating of the magnet coil which is inexpensive and easy to install.

These and other objects and advantages of this invention will become apparent from the following description wherein reference is made to the accompanying drawings, in which:

FIG. 1 is a partially sectioned perspective view of a lifting magnet in accordance with this invention;

FIG. 2 is a side view of a magnet coil assembly of FIG. I wrapped with tubing in accordance with this invention; and

FIG. 3 is a cross sectional view taken along the lines of 3-3 of FIG. 1.

Generally, the lifting magnet construction method of this invention comprises the steps of affixing lengths of air-filled tubing to an outer surface of a magnet coil, inserting and sealing the coil in a magnet case, and filling the magnet case with suitable encapsulating material.

Referring now to the drawings, a lifting magnet 11 is illustrated having a cast steel case 12. Circumferentially spaced lugs 14 are formed on the top of the magnet case 12 for attachment of hoisting chains 15 by means of pins 16.

A magnet coil assembly 17 preferably comprises one or more coils l9 wound of copper or aluminum strip of a predetermined width in stacked relation on a hub portion 20a of a bottom plate 20. Suitable electrical insulation 21 may be wound between the hub portion 200 and the coils 19 and insulation may be provided between successive turns of the coil in a manner well known to those skilled in the art. Insulating discs 22 electrically isolate the coils 19 from the magnet case 12, the bottom plate 20 and from each other. A pair of conductors 24 electrically connect the coils 19 to a direct current source (not shown) externally of the lifting magnet 11 through a terminal box 25 in the magnet case 12. The coils 19 are serially electrically connected to each other.

In accordance with this invention, prior to insertion of the magnet coil assembly 17 into a cavity formed within the case 12, one or more lengths of tubing 26 are wrapped around, and attached to, an outer surface of each coil 19. The tubing 26 may be made of any suitable material which is chemically compatible with the encapsulating material. For example, in the preferred embodiment, the tubing 26 is formed of a polyester film with a 1 inch outer diameter and a 0.010 inch wall thickness. Tubing of this nature is readily commercially available and may be obtained, if desired, in precut sections, each 6 feet long. The tubing 26 preferably has sealed ends 26a and is attached to the coils 19 by any convenient means, such as by pieces of chemically compatible adhesive tape 27. The lengths of scaled tubing 26 define air spaces therein which will be maintained circumferentially of the coils 19. Accordingly, the number of lengths of tubing 26 which will provide the proper air space to accommodate the anticipated expansion of the coils 19 can be readily selected and attached thereto.

The assembled magnet coil assembly 17, with the lengths of tubing 26 attached, is fitted into the magnet case 12 and the magnet is sealed by welding the perimeter of the bottom plate 20 to the magnet case 12. A center pole shoe 29 may be attached at the bottom of the magnet by releasable fasteners such as bolts 30 and corresponding nuts 31. The conductors 24 are passed through the terminal box 25 for external connection and the magnet is encapsulated by filling the magnet 11 with a suitable encapsulating material 32, such as tung oil compound, through any convenient opening in the magnet case 12, such as the terminal box 25, until the interior cavity of the magnet is filled and no voids remain except those controlled air spaces within the lengths of tubing 26. After encapsulation, the terminal box 25 is closed by cover plates 25a.

During operation of the magnet, a direct current source sends an electric current of relatively large magnitude through the coils 19 to produce a magnetic field for lifting billets, scrap, or other forms of magnetizable material. This magnetizing current heats the magnet coil 19 causing it to expand. The amount of radial expansion depends upon the increase in temperature which in turn is related to the duty cycle of the magnet. Without the lengths of tubing 26 in place, the expanding copper or aluminum magnet coils l9 apply an outward force to the encapsulating material 32, which, due to the lesser rate of expansion of the steel magnet case 12, causes the encapsulating material to be compressed and creates high magnitude forces within the magnet case 12. With continued heating of the magnet coils 19, the compressive forces applied to and by the encapsulating material 32 are greatly increased. These forces may cause a break in the weld or, in the case of bolted magnets, the seal between the bottom plate 20 and the magnet case 12. This may cause moisture to be drawn into the magnet cavity, as previously discussed herein, to cause insulation failure as is well known to those skilled in the art. More often, these forces cause buckling or other injury of the magnet coils 19.

This problem is eliminated by the use of the lengths of tubing 26 which provide gaps in the encapsulating material 32 near the outer surface of the magnet coils l9 and permit the coil to expand with heating so that compression of the encapsulating material 32 and the forces associated therewith are prevented. Although the same volume of air space could be provided by hollow spheres dispersed through the encapsulating material 32, the location of the air spaces cannot then be controlled so there is no way to ensure a homogeneous distribution. As previously described herein, a nonhomogeneous distribution could increase magnet damage instead of preventing it. Also, the decreased thermal conductivity of the encapsulating material caused by the introduction of the dispersed air spaces greatly detracts from its desirability in lifting magnet applications. Although the tubing 26 places air spaces at or near the surface of the magnet coils 19, adequate and even dissipation of heat through the encapsulating material 32 is assured because the location and size of the air spaces are controlled so that at least a substantial portion of the outer surface of each magnet coil 19 remains in contact with the encapsulating material 32 and a sufficient heat dissipation path to the magnet case 12 is maintained.

It should be noted that the lengths of tubing 26 described herein are disclosed by way of example and are not intended to limit the scope of the invention. Other means of providing an air space of controlled size and location at the outer surface of the coils 19 may be utilized without departing from the spirit and scope of this invention. It should also be noted that it may not be necessary that the tubing 26 be sealed or attached to the magnet coil 19 as long as it can be assured that the required air space will be provided at a controlled location generally uniformly distributed around the magnet coils after the magnet has been encapsulated.

I claim:

1. In a lifting magnet comprising a magnet case having a sealed cavity, a magnet coil within the cavity, and encapsulating material in the cavity surrounding the magnet coil, the improvement comprising means generally peripherally disposed with respect to the magnet coil for forming a compressible air gap of controlled size and location in the encapsulating material to accommodate expansion of the magnet coil and permitting at least a portion of the encapsulating material to contact the magnet coil.

2. In a lifting magnet comprising a magnet case having a sealed cavity, a magnet coil within the cavity, and encapsulating material in the cavity surrounding the magnet coil and filling portions of the cavity not occupied by the coil, the improvement comprising a compressible tube in the encapsulating material generally peripherally disposed with respect to the magnet coil and providing a compressible air space within the cavity.

3. A lifting magnet as in claim 2 wherein said magnet coil has an outer surface and including means attaching the tube to the outer surface.

4. A lifting magnet as in claim 2 wherein the tube has closed ends.

5. A lifting magnet as in claim 2 wherein the magnet coil has a predetermined width and the tube has a diameter less than said predetermined width.

k i i i t UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent IJMJ QE Dated October 2. 1973 Charles Allan Schurr It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

On the cover sheet and column 1, lines 1-3 the title should read as follows: COMPRESSIBLE AIR GAP MEANS COMPENSATING FOR TI-IERIML EXPANSION OF A LIFTING MAGNET COIL Signed and sealed this BOth dag, of April: 1971;. v

(SEAL) Attest:

EDWARD I-T.FLETCHEJR,JR. C. MARSHALL DANN Attesting Officer Commissioner 'of Patents FORM PO-IOSO (10-69) usco -pc gog' g-pag 1 us sovsrmuzm' PRINTING OFFICE mu 0-366-334,

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 'L'Z'LLLQ? Dated October 2. 1913 Inventm-(s) Charles Allen Schurr It is certified that error appears in the above-identified patent and that said. Letters Patent are hereby corrected as shown below:

On the cover sheet and column 1, lines 1-} the title should read as follows: COMPRESSIBLE AIR GAP MEANS COMPENSATIHG FOR TI'EBRIML EXPANSION OF A LIFTING MAGNET COIL Signed and sealed this 30th day of April. 19714..

(SEAL) Attest:

EDWARD ILFLETC IR IR. C. MARST'IALL DANN Attesting Officer Commissioner "of Patents Q PC4050 "$59) uscoMM-Dc 60375-P69 US. GOVERNIIENT PRINTING OFFICE 2 l9! 0"35'33, 

1. In a lifting magnet comprising a magnet case having a sealed cavity, a magnet coil within the cavity, and encapsulating material in the cavity surrounding the magnet coil, the improvement comprising means generally peripherally disposed with respect to the magnet coil for forming a compressible air gap of controlled size and location in the encapsulating material to accommodate expansion of the magnet coil and permitting at least a portion of the encapsulating material to contact the magnet coil.
 2. In a lifting magnet comprising a magnet case having a sealed cavity, a magnet coil within the cavity, and encapsulating material in the cavity surrounding the magnet coil and filling portions of the cavity not occupied by the coil, the improvement comprising a compressible tube in the encapsulating material generally peripherally disposed with respect to the magnet coil and providing a compressible air space within the cavity.
 3. A lifting magnet as in claim 2 wherein said magnet coil has an outer surface and including means attaching the tube to the outer surface.
 4. A lifting magnet as in claim 2 wherein the tube has closed ends.
 5. A lifting magnet as in claim 2 wherein the magnet coil has a predetermined width and the tube has a diameter less than said predetermined width. 